Rare earth-induced structural optimization and enhanced specific capacitance in Pr3+-doped Ni-Zn ferrites for supercapacitor electrodes

Abstract

In this study, Pr³⁺-doped Ni_0.5Zn_0.5Fe₂O₄ nanoparticles with formula Ni_0.5Zn_0.5Pr_yFe_2–yO₄ (0 ≤ y ≤ 0.125) were synthesized via the sol–gel auto-combustion method to explore the role of rare-earth substitution in tuning structural and electrochemical properties. X-ray diffraction (XRD), Raman spectroscopy, and Fourier transform infrared spectroscopy (FTIR) confirm the retention of the cubic spinel phase, while Pr³⁺ incorporation induces lattice distortion, crystallite size reduction, and cation redistribution. The Rietveld refinement analysis was performed to confirm the stable cubic spinel. Most importantly, electrochemical studies established a direct correlation between Pr³⁺ content and capacitance. The specific surface area of Ni_0.5Zn_0.5Pr_yFe_2–yO₄ was determined by using the Brunauer–Emmett–Teller (BET) theory. The optimized composition (y = 0.075) exhibits a maximum specific capacitance of 581.42 F/g at 10 mV/s, with a low charge-transfer resistance, attributed to the combined effects of an enhanced surface area, increased redox-active sites, and improved ion transport pathways. For y = 0.075, a high specific capacitance of 436.8 F/g is achieved at a current density of 2 A/g, as measured by galvanostatic charge–discharge (GCD) analysis. This enhanced electrochemical performance is attributed to the reduced crystallite size and increased surface area, which facilitate efficient charge transfer and improved electrolyte–electrode interaction. The electrochemical impedance spectroscopy (EIS) analysis further confirms the lowest R_ct (1.78 Ω) and R_s (1.78 Ω), indicating faster charge transport and enhanced supercapacitor performance. These findings demonstrate that Pr³⁺ substitution provides an effective strategy for engineering ferrite nanomaterials and underscores the broader potential of rare earth doping in developing high-performance supercapacitor electrodes for next-generation energy storage technologies.